1. Field of the Invention
The present invention relates to a ribbon microphone unit including a ribbon-shaped diaphragm (hereinafter referred to as “ribbon diaphragm”) and a ribbon microphone including the ribbon microphone unit. Specifically, the present invention relates to a ribbon microphone unit and a ribbon microphone having a structure that can prevent generation of noise affected by an external magnetic field.
2. Related Background Art
A ribbon microphone including a ribbon diaphragm is one of the magnetoelectric microphones that collect sound by electromotive force developed by vibration of a diaphragm. The ribbon microphone, which was avoided for a long time due to low sensitivity, has started to be used by preference to collect sound of voice, traditional Japanese musical instruments, and string instruments because of its soft sound quality without discomfort. The ribbon microphone has recently drawn attention in recording sites.
The ribbon microphone is provided with a ribbon diaphragm which serves as a diaphragm and a conductor in a parallel magnetic field. The ribbon diaphragm vibrates in response to sound waves in the magnetic field, and then generates power as it traverses a magnetic flux. Electrical signals proportional to a vibration speed of the ribbon diaphragm are output from two ends in the length direction of the ribbon diaphragm. The electrical signals depend on the frequency and amplitude of the ribbon diaphragm. The sound waves impinging onto the ribbon diaphragm are thus converted into the corresponding electrical signals. This is the principle of the ribbon microphone.
Such a ribbon microphone has a ribbon microphone unit as shown in FIG. 4, for example. FIG. 4 is a front view illustrating a typical conventional ribbon microphone unit. In FIG. 4, a ribbon microphone unit 101 is composed mainly of a support 102 shaped into a rectangular frame; a pair of magnets 104 mounted on the two respective long sides of the internal surface of the support 102 to generate a magnetic field; and a ribbon diaphragm 103 disposed in the magnetic field generated by the pair of magnets 104. The magnets 104 are permanent magnets and are fixed at a predetermined distance. Furthermore, the magnets 104 are magnetized in the width direction (horizontal direction in FIG. 4). Since the magnets 104 are magnetized in the same direction, a parallel magnetic field is generated between the pair of magnets 104.
Two ends in the length direction of the ribbon diaphragm 103 are fixed by electrodes (not shown in the drawing). The electrodes are in contact with electrode contacts 105 and 106, respectively, to retrieve electrical signals generated from the ribbon diaphragm 103. The electrode contacts 105 and 106 are insulated from the support 102, but are electrically conducted to the ribbon diaphragm 103 through contact with the electrodes provided on the two ends of the ribbon diaphragm 103.
FIGS. 5A to 5C illustrate only the ribbon diaphragm 103 included in the ribbon microphone unit 101 and a substrate 107 to which the ribbon diaphragm 103 is fixed. A side to which the ribbon diaphragm 103 is fixed is illustrated in a front view in FIG. 5A, in a side view in FIG. 5B, and in a rear view in FIG. 5C.
As shown in FIGS. 5A and 5B, the ribbon diaphragm 103 is sandwiched at two ends in the length direction by electrodes 115 and 116 provided on the substrate 107. The two ends are fixed by the electrodes 115 and 116 such that the ribbon diaphragm 103 is held under appropriate tension. The electrodes 115 and 116 are in contact with the electrode contacts 105 and 106, respectively. Thus, electrical signals associated with vibration of the ribbon diaphragm 103 are output through the electrode contacts 105 and 106. The ribbon diaphragm 103 is a conductor strip having a thickness of several microns. Specifically, an aluminum foil is widely used as a material for the ribbon diaphragm 103. Aluminum is suitable for a ribbon diaphragm of a ribbon microphone due to high conductivity and low specific gravity compared with other metal materials.
For use of a thin conductive plate, such as an aluminum foil, the material of the ribbon diaphragm 103 must have a reduced resonant frequency. Thus, the plate is generally corrugated as shown in FIG. 5B. The ribbon diaphragm 103 is corrugated into a bellows in the width direction (bent alternately at constant intervals into a triangular waveform). Such a bellows shape substantially lengthens the ribbon diaphragm 103, thus reducing the resonant frequency.
The ribbon diaphragm 103 described above is lightweight and easy to move and is vulnerable to noise such as human breath and external mechanical vibrations. The ribbon diaphragm 103, however, has a wide frequency range, allowing pickup of a wide range of sound from bass to treble.
FIG. 5C is a typical circuit pattern formed on the rear surface of the substrate 107 included in the ribbon microphone unit. Two signal paths 108 and 109 are provided on the rear surface of the substrate 107. The signal path 108 is integrated with the rear surface of the electrode 115 and surrounds the side surface of the ribbon diaphragm 103 over a half perimeter. Similarly, the signal path 109 is integrated with the rear surface of the electrode 116 and surrounds the other side surface of the ribbon diaphragm 103 over a half perimeter. The two ends of the ribbon diaphragm 103 held by the electrodes 115 and 116 are conductive to the signal paths 108 and 109, respectively.
With the ribbon microphone unit 101, the ribbon diaphragm 103 has the two ends fixed by the electrodes 115 and 116, which are conducted to the circuit pattern forming the signal paths 108 and 109, respectively. Thus, the electrical signals generated from the ribbon diaphragm 103 are retrieved from the electrode contacts 105 and 106 through the signal paths 108 and 109, respectively, and then are output to an amplifier circuit connected to the ribbon microphone unit 101.
The ribbon microphone including the ribbon microphone unit 101 having the structure described above is one of the electrodynamic microphones. The ribbon diaphragm 103 has low impedance, and thus a low output level. The signals output from the ribbon microphone unit 101 are thus boosted by a step-up transformer such that the output impedance is adjusted to an appropriate level prior to output.
FIGS. 6A and 6B illustrate an example of the ribbon microphone unit 101 connected to a step-up transformer 201. FIG. 6A is an example of wire connection of the ribbon microphone unit 101 and the step-up transformer 201; and FIG. 6B is an example of an equivalent circuit of the wire connection illustrated in FIG. 6A.
In FIG. 6A, the electrode 115 (not shown in the drawing) conducted to the signal path 108 is connected to one terminal on the primary side of the step-up transformer 201; while the electrode 116 (not shown in the drawing) conducted to the signal path 109 is connected to the other terminal on the primary side of the step-up transformer 201. Thus, electrical signals generated by the ribbon diaphragm 103 in response to sound waves are input to and boosted at the step-up transformer 201. The electrical signals output from the ribbon diaphragm 103 are boosted and output to the secondary side of the step-up transformer 201. The equivalent circuit of the ribbon microphone unit 101 wired as in FIG. 6A is shown in FIG. 6B. As shown in FIG. 6B, one terminal of the signal path 108 is connected to one terminal of the ribbon diaphragm 103; while the other terminal of the signal path 108 is connected to one terminal on the primary side of the step-up transformer 201. One terminal of the signal path 109 is connected to the other terminal of the ribbon diaphragm 103; while the other terminal of the signal path 109 is connected to the other terminal on the primary side of the step-up transformer 201. In the ribbon microphone unit 101, a closed circuit is thus composed of the ribbon diaphragm 103, the signal path 108, the signal path 109, and the primary side of the step-up transformer 201.
In the closed circuit shown in FIG. 6B, an external magnetic field applied around the ribbon diaphragm 103 may generate electromotive force due to electromagnetic induction in each of the ribbon diaphragm 103, the signal path 108, and the signal path 109. The electromotive forces are integrated and applied to the primary side of the step-up transformer 201. Signals generated by the external magnetic field are thus boosted by and output from the step-up transformer 201. The “signals generated by the external magnetic field” are noise. In other words, the ribbon microphone unit may generate noise affected by an external magnetic field. The external magnetic field is primarily generated by commercial AC power sources and other power sources. Although a power source is required to operate devices for amplifying and adjusting the output of the ribbon microphone (an amplifier and a mixer), such an AC power source (commercial AC power source) around the ribbon microphone generates an external magnetic field, thus generating noise.
An attempt to avoid the problem is to use a microphone case composed of a magnetic material for magnetic shielding. It is difficult, however, to completely shield an external magnetic flux. A mechanism is thus desired to cancel the induction of the external magnetic field around the ribbon diaphragm.
Another type of ribbon microphone includes four ribbon diaphragms so as to retrieve the outputs from the ribbon diaphragms separately or synthetically (refer to Japanese Unexamined Utility Model Application Publication No. S47-27831). Furthermore, a magnetic circuit of a ribbon speaker, although not a ribbon microphone, is proposed in which a magnetic flux is concentrated in a vibration system to improve conversion efficiency so as to reduce distortion of reproduced sound (refer to Japanese Unexamined Utility Model Application Publication S57-39193). In addition, a high-quality high-tone speaker is proposed in which a magnetic force of a magnetic gap is extremely large (refer to Japanese Unexamined Patent Application Publication No. 2000-350284).
None of these patent literatures, however, can prevent the noise generation by the induction of the external magnetic field.